(a) Field of the Invention
The present invention relates to a device and method for allocating a transmission opportunity to a plurality of terminals in a communication system.
(b) Description of the Related Art
Recently, the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard has been used as a communication system, particularly as a wireless local area network (LAN). The IEEE 802.11 wireless LAN is robust in terms of transmission failure. IEEE 802.11b using a 2.4 GHz industrial scientific medical (ISM) band supports a transmission speed up to 11 Mbps, and IEEE 802.11a uses orthogonal frequency division multiplexing (OFDM) in a 5 GHz band and supports a transmission speed up to 54 Mbps. In addition, the IEEE 802.11g standard that has been lately approved has a comparatively short distance, but it supports a quick transmission speed of up to 54 Mbps.
In the IEEE 802.11 wireless LAN, data are transmitted to a terminal by two methods. One of the two methods is a distributed coordination function (DCF) through a contention between terminals, and the other is a point coordination function in which a polling operation is performed by a controlling operation of a base station. In a DCF mode, all terminals may use channels with the same probability, and a priority between terminals is not considered. In a PCF mode, a central controlling polling function is used, in which a center base station controls services for the terminals. That is, the base station polls the terminals to give the terminals an opportunity for transmitting a frame.
In IEEE 802.11e, a quality of service (QoS) is provided by a hybrid coordination function (HCF) in which the DCF mode and the PCF mode in the IEEE 802.11 are combined. The QoS is guaranteed by providing respective access categories (AC) for each QoS and defining different parameters for each AC. In an HCF mode, an enhanced DCF (EDCF) increasing performance of the DCF mode according to the AC is suggested, and a transmission opportunity (hereinafter referred to as a “TXOP”) for continuously transmitting various frames without any contention for a predetermined time is provided to guarantee the QoS.
FIG. 1 shows a diagram representing the TXOP in the IEEE802.11e.
As shown in FIG. 1, a polling method is used to avoid a contention between wireless terminals. When obtaining a wireless controlling authority, a base station transmits a QoS contention-free poll (QoS CF Poll) frame to a terminal including data to transmit. The QoS CF-Poll frame includes a TXOP limit value, and the TXOP limit value is allocated to the terminal. When a polled terminal includes the data to transmit, various data frames are transmitted in the TXOP limit value. After the respective frames are transmitted, an acknowledge signal (ACK) is transmitted, and the respective frames and the respective ACKs are transmitted after a shorter interframe space (SIFS) time. In addition, a time gap may be provided in the TXOP limit value after the last frame and the ACK signal are transmitted.
In a communication system having a multi-data rate (e.g., a multi-rate IEEE 802.11e wireless LAN system), a plurality of terminals have respective data rates. A fixed TXOP according to traffic classification of the IEEE 802.11e is applied to provide the same service in the above system. However, when a high speed terminal and a low speed terminal transmit traffic requiring the same QoS requirement, different amounts of data may be transmitted for a predetermined time. For example, when the two terminals transmit data of the same priority, the amount of data to be transmitted varies according to a transmission speed for a given time due to the fixed TXOP, and therefore different service qualities may be provided according to the transmission speed of the respective terminals.